In recent years, rapid progress has been made in the design and development of ultrafast electronic and optoelectronic devices. Sampling techniques are the best method to measure fast varying electrical signals in such devices. One sampling method is the electro-optic sampling (EOS) technique which utilizes the Pockels' effect of a birefringent electro-optical crystal. However, the measurement sensitivity of the EOS technique is low due to the small change in the index of refraction with the electric field change. Better measurement sensitivity is obtainable using another method called the photoconductive sampling (PCS) technique.
The PCS technique uses a very-short-duration opto-electronic gate to map out fast electrical transients. The gate, which consists of a photoconductive gap, is usually fabricated on semiconductor materials. The photoconductive response of the material determines the sensitivity and the temporal response of the measurement. The temporal response, i.e., the measurement bandwidth, is limited because the sampled waveform is the convolved result of the real electrical signal waveform and the response function of the gate. Therefore, the shorter the gate response, the more accurately the electrical signal can be measured. The short gate response has been achieved with the fabrication of photoconductive gaps on materials having a subpicosecond lifetime of photogenerated carriers. Typical materials used are the ion-damaged silicon-on-sapphire and the low-temperature-grown GaAs. The short-carrier-lifetime has been attained using defects in the material. The defects reduce the mobility of carriers, resulting in the decrease of the measurement sensitivity, although the decreased sensitivity is still much higher than that of the EOS technique.
The impulse samplers have been used to characterize high frequency microwave and millimeter wave devices and circuits. Since these high speed devices and circuits can have frequency responses over 100 GHz, which conventional electronic measurement systems cannot reach, optical measurement techniques such as photoconductive sampling have been successfully adopted for the analysis of such devices and circuits.
The conventional PCS technique using a short-duration gate can be described as follows. A series of a repeating voltage signal is represented by f(t). If f(t) is the voltage signal to be sampled and h(t) is the rectangular response function of the sampling gate due to an optical signal driving the sampling gate, and these two signals are perfectly synchronized, then the measured signal F(t) is the cross-correlation between f(t) and h(t): EQU F(t)=.intg..sub.-.sup..infin..sub..infin. f(t')h(t'-t)dt'. (1)
For a rectangular gate-function h(t) given by EQU h(t)=h.sub.0, for 0.ltoreq.t.ltoreq..tau. EQU h(t)=0, for t&lt;0 or t&gt;.tau., (2)
the signal F(t) becomes ##EQU1## If the gate duration .tau. is small enough compared to the signal under study f(t), the signal that is measured F(t) is given by EQU F(t).apprxeq.h.sub.0 .tau.f(t) (4)
and F(t) is proportional to the signal under study f(t).
As such, in order to have a large F(t), it is necessary to have a large h.sub.0 while maintaining a large .tau.. Here, h.sub.0 is the sensitivity of the photoconductive gate response and is proportional to the mobility of the material which is very material dependent. Because h.sub.0 is usually in a trade-off relationship with the gate recovery time .tau., it is almost impossible to get high sensitivity with picosecond time resolution in a conventional photoconductive sampling circuit. Thus, a need exists for a technique that has very high measurement sensitivity but does not sacrifice the measurement time resolution.